Slope aspect information is widely used by earth scientists, environmental planners, and other analysts to represent a physical landscape. A terrain surface is commonly subdivided into an array of contiguous surface elements in which the position of each of these surface elements is defined by data specified in a two dimensional or three dimensional positional coordinate system. In addition to its positional coordinates, each surface element may be defined by a further parameter such as its slope aspect. The slope aspect may be generally defined as the compass direction toward which the maximum slope of the surface faces and more particularly may be defined as the compass direction or azimuth of the horizontal component of the gradient which is a line along the maximum slope of the surface element.
Although aspect is a continuum extending around the entire 360 degree universe of possible azimuths, it has long been deemed desirable to subdivide that 360 degree universe into a finite plurality of contiguous, angular aspect intervals or classes. For example, the 360 degrees may be subdivided into eight intervals of 45 degrees each or 16 intervals of 22.5 degrees each.
Slope aspect is a landscape characteristic which is fundamental to building site analysis, solar access planning, water shed management, and many other scientific and management activities. Although determining the aspect at a single location may be sufficient for addressing some problems, most problems require an understanding of the pattern of slope aspect variation across the landscape. Slope aspect maps provide such a desirable regional view and are required in many instances.
Slope aspect maps are frequently created using computers because aspect computation based upon an array of grid cells or elements in a digital elevation model is a straight forward and an efficient procedure which can be calculated from elevations of adjacent cells.
Cartographers have long used grey tones for standard terrain shading to illustrate the form of the terrain. This method simply determines the slope gradient normal for each surface element, assumes a light origin azimuth, conventionally 315 degrees, and then illuminates each element in a relatively lighter or darker grey tone as a function of the assumed illumination on the surface element as determined by its slope gradient normal, that is its angle relative to the light source.
One worker in the prior art has extended these principles to the presentation of slope aspect data utilized a cosine equation of illumination to determine the grey tone used to display each element as a function of aspect. If a grey scale is used with different grey tones, then the aspect of each element is displayed by the grey tone which represents the interval in which the element's aspect lies.
Although such a presentation of the data with such grey tones does permit general qualitative visualization of the surface slopes, the grey tones are perceived as implying magnitude variations. That is not appropriate for a display of slope aspect data.
Prior art workers have attempted to overcome this problem by using the hue component of color to distinguish aspect classes or intervals. In order to uniquely define each aspect interval, prior art workers have assigned a different hue to each of the selected subdivided aspect intervals and then displayed each element by the hue assigned to the aspect interval in which that element's aspect lies. These prior art attempts at using hue have been successful in presenting the data in a manner in which the aspect interval or class for each element may be easily distinguished and determined. The spectral hues of red, orange, yellow, green, blue, violet, and mixtures thereof produce, on some maps, easily distinguishable classes in the map legend and within the map. These hues at varying levels of value and chroma have been randomly assigned to classes. Yellow and neighboring lighter hues have been used to represent virtually all aspect directions.
The difficulty with these prior art attempts is that, although they facilitate discrimination of the aspect classes, they do not present a map from which a human can perceive some correspondence between the display and the form of the terrain being represented.
Since the use of grey tones in the prior art has enabled the human visualization of underlying form from the display and since the presentation of colors have permitted the discrimination between slope aspect class data presented on a map, we made attempts to combine these two by overlaying or over printing the aspect colors on a relief shaded base map. The results, however, were disappointing because the presence of the grey tone shading inherently decreases the user's ability to distinguish among the classes and the presence of the variety of colors is confusing with respect to the relief displayed by the grey tone.
As a result, it is the purpose of the present invention to present a method for displaying aspect information in a manner which both allows easily perceived and accurate discrimination of individual classes or intervals of slope aspect while simultaneously facilitating the user's visualization of the underlying surface form. This enables the map user to easily determine the aspect class while implicitly perceiving the slope from the same display.
Thus, it is the principal object of the present invention to develop an element coloring scheme that will maximize color contrast among aspect interval classes while assisting the user to visualize the form of the underlying landscape.
The present invention draws upon the teachings of the opponent process color theory as explained, for example, in an article by J. R. Eastman in The American Cartographer, Vol. 13, No. 4, 1986, pp. 324-333.